In general, a lithographic printing plate is composed of an oleophilic image area accepting ink in the process of printing and a hydrophilic non-image area accepting dampening water Lithographic printing is a printing method comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink unreceptive area), thereby making a difference in adherence of ink on the surface of the lithographic printing plate, depositing the ink only to the image area by utilizing the nature of the dampening water and oily ink to repel with each other, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then removing the image-recording layer in the non-image area by dissolving with an alkaline developing solution or an organic solven thereby revealing the hydrophilic surface of support while leaving the image-recording layer in the image area.
In the hitherto known plate-making process of lithographic printing plate precursor, after exposure, the step of removing the non-image area by dissolving, for example, with a developing solution corresponding to the image-recording layer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate-making methods in response to the above-described requirement, it has been proposed a method referred to as on-machine development wherein a lithographic printing plate precursor having an image-recording layer capable of being removed in a conventional printing process is used and after exposure, the undesirable area of the image-recording layer is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-machine development include, for example, a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-recording layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-recording layer or adhesion between an image-recording layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-recording layer by contact with rollers or a blanket cylinder of a printing machine.
In the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing a laser-unexposed area in the lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developing solution) thereby revealing a bydrophilic surface of support. The term “on-machine development” means a method and a step of removing a laser-unexposed area in the lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, laser light and conducting scanning exposure of a lithographic printing plate precursor with the light thereby directly preparing a lithographic printing plate without using a Jith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
As a lithographic printing plate precursor capable of undergoing scanning exposure among the lithographic printing plate precursors, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer containing a photosensitive compound capable of generating an active species, for example, a radical or a Bronsted acid by laser exposure has been proposed and such a printing plate precursor has been put on the market. A negative lithographic printing plate can be obtained by subjecting the lithographic printing plate precursor to laser scanning on the basis of digital information to generate an active species, causing a physical or chemical change in the photosensitive layer by the action of the active species to make the photosensitive layer insoluble, and then development processing the lithographic printing plate precursor. Particularly, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a photopolymerization type photosensitive layer containing a photopolymerization initiator excellent in photo-speed, an addition polymerzable ethylenically unsaturated compound and a binder polymer soluble in an alkali developing solution and, if desired, an oxygen-blocking protective layer can provide a lithographic printing plate having desired printing properties, for example, excellent productivity, ease of undergoing development processing, and good resolving power and ink receptivity.
As a lithographic printing plate precursor capable of undergoing on-machine development, for example, lithographic printing plate precursors having provided on a hydrophilic support, an image-forming layer in which hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder are described in Patent Document 1. It is described in Patent Document 1 that the lithographic printing plate precursor can be exposed to an infrared laser to agglomerate the hydrophobic thermoplastic polymer particles by heat thereby forming an image, and mounted on a cylinder of a printing machine to carry out on-machine development by supplying dampening water and/or ink.
Although the method of forming image by the agglomeration of fine particles only upon thermal fusion shows good on-machine development property, it has a problem in that the image strength (adhesion to the support) is extremely weak and printing durability is insufficient.
Further, lithographic printing plate precursors having provided on a hydrophilic support, microcapsules containing a polymerizable compound encapsulated therein have been proposed (for example, see Patent Documents 2 and 3).
Moreover, lithographic printing plate precursors having provided on a support, a photosensitive layer containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound have been proposed (for example, see Patent Document 4).
The methods using the polymerization reaction has a feature that since chemical bond density in the image area is high, the image strength is relatively good in comparison with the image area formed by the thermal fusion of fine polymer particles. However, from a practical standpoint, any of the on-machine development property, printing durability and polymerization efficiency (sensitivity) is still insufficient and the methods have not been put into practical use.
Patent Document 1: Japanese Patent 2,938,397
Patent Document 2: JP-A 2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 3: JP-A-2001-277742
Patent Document 4: JP-A 2002-287334